3GPP LTE (3rd Generation Partnership Project Long Term Evolution) is the upcoming standard of 4th generation radio access networks. A continuing major challenge for designers is to reduce the system power consumption of mobile terminals, also known as User Equipment (UE) in LTE terminology. In recent communication schemes like UMTS and the upcoming LTE system, UEs use a Discontinuous Reception (DRX) mode.
DRX means that the UE discontinues monitoring the Physical Downlink Control Channel (PDCCH) for well-defined sub frames (e.g. 2 ms per 20 ms) and is allowed to enter a power saving mode. FIG. 1 shows the basic principle of DRX functionality. The DRX procedure is defined by a set of UE specific parameters that are configured via Radio Resource Control (RRC) signaling. The RRC protocol has two states: RRC_IDLE and RRC_CONNECTED. The RRC_CONNECTED state means that the respective UE is known to the network, the UE's location is known on cell level, mobility is controlled by the network, unicast data transfer is possible, and that the DRX mode is supported for power saving.
The DRX procedure basically consists of DRX cycles that have an ‘on’ duration where the UE shall monitor the PDCCH for dynamic schedules, and a DRX period where the UE is allowed to discontinue monitoring the PDCCH and may go to a power save mode.
FIG. 2 illustrates a protocol stack of a UE according to LTE standard specifications in the terms of the OSI (Open Systems Interconnection) model of logical layers. As is known in the art, the Non-Access Stratum (NAS) layer protocol is responsible for signalling and traffic between UE and the network for control purposes such as network attach, authentication, setting up of bearers, and mobility management. The Radio Resource Control (RRC) protocol of layer 3 is responsible for control plane signalling between a UE and the network, i.e. such tasks as broadcast of system information; establishment, maintenance and release of RRC connection; establishment, configuration, maintenance and release of signalling and data radio bearers; security functions including key management; mobility functions such as control of UE cell selection/reselection; paging; UE measurement configuration, processing and reporting; handover; quality of service (QoS) management functions; UE measurement reporting and control of the reporting, but not exclusively. Layer 3 interfaces with layer 2 and also directly interfaces with layer 1. Layer 2 of the protocol stack comprises a Packet Data Control Protocol (PDCP) sub-layer which is responsible for (de-) compressing the headers of user plane IP packets, a Radio Link Control (RLC) sub-layer which is used to format and transport traffic, and a Medium Access Control (MAC) sub-layer which provides addressing and channel access control mechanisms. Layer 3 interfaces with layer 2 and layer 1. Layer 1, also known as physical (PHY) layer, translates logical communication requests into hardware-specific operations such as modulation, bit synchronization, multiplexing, equalization, forward error correction etc. It has to be noted in this context that typical physical UE implementations have the stack controller and the physical (PHY) layer running on separate hardware blocks that can enter power saving modes independently. In other words, the aforementioned logical layers of a single UE are typically implemented in hardware as separate systems-on-a-chip (SoCs) which might even be provided by different vendors.
According to LTE protocol standard specifications, the DRX functionality is controlled by the MAC layer, i.e. the MAC layer is responsible for controlling DRX cycles. It has to wake up prior to an ‘on’ duration in order to wake up layer 1.
However, when implemented in the MAC layer as per definition of the standard, some inefficiency with respect to power consumption is system immanent:                The MAC layer does not have the exact knowledge of the timing of the physical layer (PHY), and therefore it has to schedule wake-up prior to an ‘on’ duration well in advance to ensure full operation timely.        Due to the latencies between MAC and PHY layers, the PHY layer and especially the transceiver thereof, is turned off with significant latency compared to the opportunity as seen on the air interface.        The MAC layer, and hence the entire hardware block that implements layer 2 or the entire stack controller, is active for ‘on’ durations that expire without any activity.        
A general object of the invention is to improve the system power consumption of user equipment (UE) of a wireless communication system that is running the DRX mode in RRC_CONNECTED state.